Magnetic recording media have a magnetic layer containing magnetic particles and a binder as main components. The magnetic layer is coated on a nonmagnetic support, such as a polyethylene terephthalate, polyethylene naphthalate, a polycarbonate or a polyamide film.
The magnetic particles are a key component of the magnetic recording media. Magnetic particles present in the magnetic layer typically are acicular particles having magnetic moments capable of responding to an externally applied magnetic filed. The ability of magnetic particles to respond to an externally applied magnetic field in a desired way is measured by several established parameters. Four important parameters are remanence (B.sub.r), coercivity (H.sub.c), switching field distribution (SFD) and squareness (Sq). These four parameters measure, respectively, the extent to which a magnetic particle remains magnetized after the external applied field is removed (B.sub.r), the ability of the imposed magnetization to be maintained rather than lost through the influence of adjacent domains (H.sub.c), the measure of the spread of coercivities around an average value (SFD) and the ability of the recording medium to retain the recording signal in memory (Sq). These parameters are determined using standard methods, and are used to measure the quality of recording that can be obtained.
A variety of magnetic particles have been used in the magnetic layer of magnetic recording media. The usefulness of a magnetic particle is related to the chemical composition, size, shape and magnetic properties of the particle. For example, desirable magnetic particles have a high coercivity and a high remanence to give high output; and have a small particle size to provide smoother surfaces (i.e., increased output) and lower noise. However, in practical applications, compromises are required. For example, record and erase head limitations place an upper limit on the coercivity, and magnetic or chemical stability places a lower limit on the reduction of particle size.
The switching field distribution (SFD) is an important magnetic property of magnetic particles used in magnetic recording media and is related to coercivity. Coercivity (H.sub.c) is the field strength required to cause a magnetic reversal of the magnetic particles in the magnetic recording media. The coercivity is sufficiently small to allow successful writing, and overwriting or erasure, by available heads. The coercivity also is sufficiently large for the magnetic particles to resist unwanted changes or degradation of the signal during storage. The coercivity however is only a median field strength, and the breadth of the distribution of field strength, centered on the coercivity, wherein magnetic reversal occurs, also is important. The breadth of this distribution is the SFD.
A narrow SFD facilitates the writing of sharp, well-defined magnetic transitions and therefore improves the ability of magnetic particles to record information at high densities. A broad SFD not only diffuses the magnetic transitions but also can lead to a variety of other problems, such as problems in erasure or in overwriting old information with new information. Therefore, in addition to providing a magnetic particle having a sufficient coercivity, the magnetic particle also must have a sufficiently narrow SFD.
Magnetic particles or pigments used in magnetic recording media are prepared by a chemical precipitation process. Chemical precipitation provides small magnetic particles of uniform composition and having a narrow size distribution. The particle size and morphology of precipitated magnetic particles can be controlled by varying precipitation conditions, such as concentration of starting materials, temperature, reaction time, and choice of starting materials.
Historically, gamma ferric oxide (.gamma.-Fe.sub.2 O.sub.3) was the standard magnetic particle used in magnetic recording media. Gamma ferric oxide has excellent chemical and physical stability and therefore has been the most useful of the magnetic particles. The presently-used .gamma.-Fe.sub.2 O.sub.3 particles are acicular (i.e., needle-like or rod-like) in shape. This shape anisotropy is a major source of the magnetic anisotropy of acicular .gamma.-Fe.sub.2 O.sub.3. Another source of magnetic anisotropy in .gamma.-Fe.sub.2 O.sub.3 is magnetocrystalline in origin, arising from the interaction of electron spins with the crystal structure of the oxide. These two types of anisotropy determine the external magnetic field needed to switch the magnetization from one energetically preferred direction to another preferred direction, and therefore determine the coercivity of the magnetic particle.
Acicular .gamma.-Fe.sub.2 O.sub.3 particles typically have a length of about 0.15 to about 0.50 .mu.m (micrometers). Magnetic particles of this size are a compromise between being sufficiently small to assure a low noise level and sufficiently large to avoid magnetic instability. Acicular .gamma.-Fe.sub.2 O.sub.3 particles having a coercivity in the range of 300 to 400 Oe (Oersteds) are still used in present-day low-density magnetic recording media and are precursors for modified magnetic iron oxides. These magnetic particles have coercivities up to about 1000 Oe and are more suitable for use at high recording densities, but also are more expensive and in some respects less stable.
Gamma ferric oxide is produced by first growing particles of iron oxide hydroxide, either .alpha.-FeOOH (goethite) or .gamma.-FeOOH (lepidocrocite). Dehydration of .alpha.-FeOOH forms particles of nonmagnetic .alpha.-Fe.sub.2 O.sub.3 (hematite). The hematite particles then are reduced to yield particles of Fe.sub.3 O.sub.4 (magnetite). Magnetite itself is a useful magnetic particle, and has served as the core particle for a cobalt surface-doped magnetic particle disclosed in Kanten U.S. Pat. Nos. 4,137,342 and 4,226,909. Magnetite, however, has chemical and magnetic instabilities, and therefore is oxidized to particles of .gamma.-Fe.sub.2 O.sub.3 for most recording applications.
Gamma iron oxide, having a coercivity of about 300 to about 400 Oe, is suitable only for low recording density applications. However, many magnetic recording applications require high recording densities, and therefore require a magnetic particle having a coercivity of greater than about 400 Oe. Cobalt-doped iron oxide particles have been used in these high recording density applications.
Cobalt-doped iron oxides exhibit many of the benefits of .gamma.-Fe.sub.2 O.sub.3 (e.g., chemical stability), and also have a coercivity of about 400 to about 1000 Oe. Early cobalt-doped iron oxides had cobalt(II) ions uniformly dispersed throughout the iron oxide particle. These cobalt-doped iron oxides exhibited a temperature-dependent coercivity and were subject to mechanical stresses that reduced magnetic properties.
Improved cobalt-doped iron oxides positioned cobalt(II) on or near the surface of the magnetic particle. These doped magnetic particles were termed "epitaxial" or "surface-doped" products, and demonstrated an increased coercivity and a decreased temperature dependence over the early cobalt-doped iron oxides. Cobalt-modified iron oxide particles have been technically and commercially successful in magnetic recording media. However, investigators continue to seek improved doped magnetic particles that have higher coercivities and narrower switching field distributions.
Cobalt-doped iron oxides, and methods for preparing them, have been disclosed in several patents and publications. For example, Kanten U.S. Pat. Nos. 4,137,342 and 4,226,909 disclose cobalt-doped iron oxides, and the preparation thereof, wherein cobalt(II) is present in the surface layer over a core including ferrous and ferric iron.
Schwab et al. U.S. Pat. No. 4,770,903 discloses a magnetic iron oxide having a .gamma.-Fe.sub.2 O.sub.3 core and a ferrite shell including cobalt(II) and iron(II). The magnetic iron oxide is prepared by performing two precipitations.
Ogawa et al. U.S. Pat. No. 4,863,793 discloses ferromagnetic iron particles, including cobalt-modified particles. The ferromagnetic particles further can include elements, such as silicon, to improve the properties of the particles.
European Patent Application 0 393 563 discloses cobalt-modified iron oxide particles having a magnetic core and a ferrite shell including cobalt(II) and iron(II). The cobalt-modified iron oxide particles, after doping, are treated with an aqueous alkali silicate to improve the stability of the cobalt-modified iron oxide.
Japan Laid-Open Patent Application JP H2-167829 discloses a cobalt-containing ferromagnetic iron oxide powder wherein a silicate compound and a cobalt compound are coated on acicular alpha iron oxide hydroxide (.alpha.-FeOOH).
Japan Laid-Open Patent Application JP H1-188428 discloses a cobalt-containing ferromagnetic iron oxide powder and its method of manufacture wherein the powder has a nuclear crystal covered with a cobalt compound. The outermost layer of the powder also can include silica.
The above-described references disclose a method of providing a layer of cobalt(II) and iron(II) on the surface of an iron oxide core. However, such cobalt-doped magnetic particles exhibit disadvantages and drawbacks, such as a high switching field distribution (SFD) and a relatively high coercivity instability in relation to changes in temperature. The present invention therefore is directed to: 1) a doped magnetic iron oxide particle that overcomes these disadvantages; and 2) to methods of manufacturing the doped magnetic iron oxide particle.